Physical Attributes of Sand Bodies Within High-Frequency Stratigraphic Sequences

Recently, the stratigraphic concepts of genetic depositional systems (Fisher and McGowen, 1967), systems tracts (Brown and Fisher, 1977), genetic sequences (Frazier, 1974; Galloway, 1989), and seismic sequences (Vail et al., 1977) have been modified and merged to gain a better understanding of the stratal relationships and spatial distribution of both elastic and carbonate sediments using sequence stratigraphic models (Posamentier et al., 1988). These broad conceptual models have primarily been used to analyze the thick fill of sedimentary basins on passive continental margins and to develop regional stratigraphic frameworks emphasizing the global influence of eustatic sea-level fluctuations. Analytical techniques are just now being developed to combine the mapping of seismic sequences with borehole data (electric logs, sediment velocities, and sediment bulk densities) so that lithologies can be predicted within depositional sequences and potential reservoir-quality sand can be located relative to sequence boundaries. Correctly interpreting the presence or absence of sand in elastic depositional sequences on the basis of seismic response represents a key element to sequence stratigraphic studies. These concepts are especially important in offshore or other frontier exploration areas far removed from outcrops or other geological information that would allow forward prediction of subsurface lithologies. Geophysicists using conventional common-depth-point (CDP) data are able to derive general lithologic properties from relative impedance and seismic velocity (Savit and Wu, 1982). Although these derivation techniques are constantly being improved, the pre-drilling prediction of lithologies still relies heavily on seismic facies interpretations as well as a general understanding of sedimentary processes and geological history of the basin fill.